TAR DNA-binding protein 43 kDa (TDP-43) is the major aggregating disease protein in amyotrophic lateral sclerosis (ALS). Over 90% of ALS cases exhibit pathological lesions containing detergent insoluble deposits of phosphorylated, truncated, and ubiquitinated TDP-43 protein. TDP-43 phosphorylated at S409/410 (pS409/410) is the most consistent, robust, and specific neuropathological feature of ALS suggesting a phosphorylated TDP-43 (pTDP) mediated cascade of neurotoxicity. Furthermore pTDP has been shown to influence the aggregation of TDP-43 in cultured cells and in human ALS cases. Our previous work demonstrated pS409/410 TDP-43 mediates motor neuron toxicity of familial ALS-causing TDP-43 mutations. Kinases regulating TDP-43 phosphorylation present an attractive target for therapeutic intervention in ALS. We have identified a well-conserved TDP-43- active kinase with translational potential known as tau tubulin kinase 1 (TTBK1). Identification of brain penetrant TTBK1 inhibitors may ultimately provide a viable drug development strategy. We hypothesize that increased TDP-43 phosphorylation drives motor neuron degeneration in ALS and that blocking pTDP accumulation by inhibiting TTBK1 will protect against TDP-43 mediated neurodegeneration in ALS. Three integrated specific aims are proposed: 1) Identification of TTBK1 selective kinase inhibitors. 2) Optimization of TTBK1 selective inhibitors and validation of selective compounds in a cellular model of pTDP accumulation. 3) Validation of the role of TTBK1 in the formation of pTDP using TTBK1 knockout mice and an existing transgenic model of TDP-43 proteinopathy. The studies proposed here will set the stage for development of TTBK1 selective inhibitors as a candidate therapeutic approach for ALS.